Ancient Soil Samples Link Fires to Climate Change

Those who cannot remember the past are condemned to repeat it. In the case of global warming, that can have dangerous, smoldering-hot implications. Global surface temperatures are expected to go up at least a degree over the next century, and the last time that happened, in the Middle Ages, the frequency of wildfires in the Colorado Rockies nearly doubled.

A number of recent studies suggest the weather is likely to get a lot harsher in the decades and centuries to come, with more intense (though not necessarily more frequent) hurricanes, longer and more severe droughts, and increasing numbers of severe thunderstorms. More droughts in particular also means a greater risk of wildfires. Less water, after all, equals more fire. But just how bad are things likely to get?

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Even within the past 20 or 30 years, rising temperatures have led to increasingly large wildfires.

The outlook's not too rosy, according to John Calder, a University of Wyoming geophysics graduate student, and his colleagues. Their findings, published today in Proceedings of the National Academy of Sciences, "indicate a significant risk that ... fires will burn large areas in the coming century if temperatures continue to rise," they write.

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That conclusion is based not on climate models or recent trends in forest fires, but rather on records of forest fires that occurred more than a millennium ago, during the Medieval Climate Anomaly, a period when global temperatures were comparable to what they are today, and about half a degree warmer (on the Celsius scale) than they had been for several centuries prior. To see if that unusually warm period had any effect on the frequency of forest fires, Calder and his team collected sediment core samples from 12 lakes in the Mount Zirkel Wilderness in northern Colorado. Layers of charcoal within those core samples, the researchers point out, show not only that fires had burned nearby, but also approximately what year the fire occurred.

The charcoal records revealed that the frequency of fires went up substantially during the MCA. Typically, 10 out of 12 sites burned per century, compared with six per century before the MCA, and four per century after. More importantly, that uptick was strongly correlated with temperatures—as temperatures rose, fire frequency went up, and as they fell back down, so too did fire frequency.

"Historically, few landscapes burned as extensively as the Mount Zirkel Wilderness burned during the MCA, but [modern] temperatures have only been comparable to the MCA for the past few decades," the researchers write. Yet even within the past 20 or 30 years, rising temperatures have led to increasingly large wildfires. As those temperatures continue to climb, it's likely we'll be in for a real scorcher in more ways than one.

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